1. Field of the Invention
This invention relates to a pixel control circuit, and especially relates to a pixel control circuit that is able to avoid a brightness of the pixel from being affected by characteristics of transistors.
2. Description of the Prior Art
FIG. 1 shows a pixel control circuit 100 according to prior art. The pixel control circuit 100 includes a switch T1A, a driving transistor T1B, a capacitor C1, and an organic light emitting diode 110. The switch T1A has a first terminal for receiving a data signal Sdata, a second terminal, and a control terminal for receiving a scan signal Sscan. The driving transistor T1B has a first terminal for receiving a system voltage OVDD, a second terminal coupled to a first terminal of the organic light emitting diode 110, and a control terminal coupled to the second terminal of the switch T1A. The capacitor C1 has a first terminal for receiving the system voltage OVDD, and a second terminal coupled to the control terminal of the driving transistor T1B.
When the switch T1A is turned on by the scan signal Sscan, the driving transistor T1B can conduct a current IOLED according a voltage of the data signal Sdata to turn on the organic light emitting diode 110. The current IOLED can be represented as IOLED=K(VSG−|VTH|)2 according to characteristics of the transistors, where K represents a manufacturing parameter of the driving transistor T1B, VSG represents a source-to-gate voltage of the driving transistor T1B, and VTH represents the threshold voltage of the driving transistor T1B. In FIG. 1, the driving transistor T1B is a P-type metal-oxide-semiconductor transistor, and the source-to-gate voltage VSG is the system voltage OVDD minus the voltage of the data signal Sdata.
Consequently, although the pixel control circuit 100 may control the strength of the current IOLED flowing through the organic light emitting diode 110 according to the voltage of the data signal Sdata, each of the pixels in a display may still have different brightness even if each of the pixels shows a brightness according to the same data signal Sdata, due to different characteristics of transistors in each of the pixels, which may cause luminance nonuniformity. This is because the threshold voltage VTH of the driving transistor T1B may be affected by the manufacturing process or may be changed after being used for a long time. Thus, the image quality may also drop as the time goes by.
Furthermore, since the pixels are disposed in different positions of the display, the system voltage OVDD received by each of the pixels may also be different due to different levels of resistance of the OVDD transmission line, which makes it even more difficult to control the luminance uniformity.
In addition, since the pixel control circuit 100 does not provide the organic light emitting diode 110 with any discharge path, there may be some charges remaining in the organic light emitting diode 110 after a previous image is over. Therefore, if the following image is a black image, the display may have the issue of not being dark enough while displaying the black image.